1. Field of the Invention
The present invention relates to a method for evaluating sensitivity of a photoresist used in a photolithography process for manufacturing a semiconductor device, a method for preparation of a photoresist and a manufacturing method of a semiconductor device.
2. Description of the Related Art
In a photolithography process for manufacturing semiconductor device such as a semiconductor integrated circuit, a circuit pattern delineated on a photomask is transferred onto a resist film coated on a semiconductor substrate by an exposure tool. In accordance with recent higher integration and higher performance of a semiconductor device, it is required to transfer a finer circuit pattern delineated on a photomask to a semiconductor substrate. The imaging performance of a reduction projection exposure tool can be evaluated using an optical imaging theory derived from the Rayleigh equation. A resolution R of an exposure tool is proportional to a wavelength λ of the exposure light and inversely proportional to a numerical aperture NA. Furthermore, a depth of focus DOF is proportional to the exposure wavelength λ and inversely proportional to the square of the numerical aperture NA. Accordingly, in response to requirements for miniaturization of a semiconductor device, improvements in the process have been achieved, including shortening of the exposure wavelength λ and achieving a higher NA of a projection lens. However, along with the recent demands for further miniaturization of the semiconductor device, it has become extremely difficult to ensure an exposure latitude and a depth of focus DOF. Thus, the miniaturization of a semiconductor device significantly reduces the manufacturing yield.
To implement a photolithography process with a small process margin, great importance has been attached to a precise analysis of errors that decrease the process margin and error allocation (error budget). For example, even when many chips on a semiconductor substrate have been exposed with the same prescribed exposure dose, the effective appropriate exposure dose varies due to avariation in photoresist sensitivity, post exposure bake (PEB), nonuniformity of development in a substrate, a variation in thickness of a resist film on a substrate, and the like. Consequently, a decrease in manufacturing yield occurs. To efficiently apply the small process margin and to prevent a decrease in the manufacturing yield, a method for implementing feedback or feed forward control by highly accurate monitoring an exposure dose and focusing is needed. At the same time, it is also necessary to perform a precise analysis on error factors which decrease the process margin for each process unit and to correct the major error factors based on results of the analysis.
For controlling an exposure dose and focusing in a reduction projection exposure tool, an exposure dose monitoring method and a focus monitoring method have been proposed (refer to Japanese Patent Laid-Open No. 2001-102282). In the proposed exposure dose monitoring method, an image is transferred with a stepped exposure distribution by using a photomask having a pattern therein with a dimensional ratio (a duty ratio) of a transparent portion and an opaque portion that is continuously changed in one direction by a pitch that can not resolve images on the semiconductor substrate. In the proposed focus monitoring method, focus is monitored using a focus monitor mark having diamond-shaped patterns that shift each phase of the exposure light so as to show different characteristics of pattern dimensions for defocus.
In addition, as a sensitivity control method for a photoresist, the sensitivity is evaluated by measuring a dimension of a delineated pattern on a resist film, which is transferred by a prescribed mask pattern using an exposure tool. Another sensitivity control method for a photoresist has been provided, where the sensitivity is evaluated by determining a clearing dose by a pattern sufficiently larger than a minimum feature size corresponding to a resolution of an exposure tool. Here, the “clearing dose” refers to the lowest exposure dose for which all of the resist film is removed.
As described above, in microfabrication it is important to control exposure conditions for photolithography highly accurately in order to achieve processing accuracy and uniformity in pattern dimensions of semiconductor devices. Particularly, an evaluation method for photoresist sensitivity becomes more important for a fine pattern with smaller exposure latitude and smaller focus margin, or for an isolated fine pattern. However, in the evaluation method for photoresist sensitivity by measuring a dimension of a delineated pattern, the dimension may vary depending on not only the exposure dose but also focusing of an exposure tool. Therefore, using the method for measuring the dimension of the delineated pattern, it is difficult to determine variations in the photoresist sensitivity, separating from the focus variation. Furthermore, with regard to the evaluation method by determining the clearing dose, it is difficult to determine an accurate exposure dose corresponding to the remaining thickness of the resist film, since the remaining thickness changes abruptly in the vicinity of the clearing dose.